This invention relates to projection screens for rear projection television receivers and in particular, to screens having a plurality of lens element layers.
Rear projection television receivers are made up of one or more display tubes or kinescopes, a lens and reflecting mirror arrangement, and a translucent screen which forms an enlarged image of the kinescope display screens. The images from the kinescopes are projected onto the rear of the screen, permitting the entire projection television structure to be enclosed in a single closed cabinet.
A representative rear projection television screen comprises a circular Fresnel lens which substantially collimates the light from the kinescope display screens. A lenticular lens array, composed of many vertically oriented lenticular lenses, focuses the light rays from the Fresnel lens onto an image plane, which may be part of the lenticular array. The lenticular lenses control the horizontal dispersion angle, which defines the horizontal audience envelope. A diffusion layer, which may be a coating on one surface of the lenticular lens array, provides a vertical dispersion angle defining a vertical audience envelope. The audience envelope is defined as the angle between light rays which are 3 dB down from the maximum ray, e.g., rays having 50% brightness as compared to the maximum brightness ray.
The vertical audience envelope provided by the diffusion layer is of the order of 15.degree.. The most desirable viewing position therefore requires that the viewer's eye level be substantially in line with the principal ray axis, which is ordinarily coincident with the center of the screen. This position may be difficult for viewers to achieve in some installations, such as when the receiver is positioned above the viewers so that a large number of people may watch a single screen.
It is also desirable to provide a compact, low profile receiver for home use in order to minimize the disruption of room decor. Such a low profile projection television receiver results in the principal ray axis of the projection screen originating substantially below the eye level of a normal viewer, so that many viewers will be out of the desired audience envelope.
Several solutions to this problem exist. The entire projector mechanism including the screen may be tilted back. This, however, subjects the screen to reflections from overhead lights. Leaving the screen vertical while tilting the remaining projector mechanism will also change the angle of the principal light ray, but this arrangement introduces undesirable keystone distortion in the scanned rasters.
A special diffuser may be used to introduce an upwardly biased diffusion component. This, however, requires an additional screen element in the screen assembly. A prism component may also be built into the screen assembly to provide a bias to the light rays. Although this does not require an additional screen element, the existing elements become more complex and expensive.